One type of solar collector includes a concentrator, such as a parabolic dish reflector, which forms a concentrated image of the sun onto the opening of a receiver. The concentrated solar radiation heats tubes within the receiver chamber that carry water or other working fluid. If the concentrated image of the sun strikes the receiver face plate, due to a tracking failure or a major error in alignment, then the face plate may be burned. Similarly, if working fluid in the receiver tubes is lost, or circulation stops because of a pump failure, the tubes can be damaged because of overheating. A relatively simple apparatus that could protect the receiver against damage in the event of such failures, while avoiding interference with normal operation of the concentrator system, would be of conconsiderable value.